This invention relates to a meter for measuring fluid flow by detecting Karman vortices, In particular, the invention is an improved vortex flowmeter that compensates for changes in the Strouhal Number, dimensional changes of a bluff body, or for measurement error associated with conditions in a pipe which distort the fluid velocity profile.
The accuracy of vortex flowmeters depends in part on using a proper value for the Strouhal Number, N.sub.s. The Strouhal Number is a non-dimensional flow number that is related to the stability of fluid flow when an obstruction is placed in the flow. Once the Strouhal Number is determined or calibrated for a particular vortex flow meter, it typically remains constant when the meter operates. This means that the Strouhal Number can usually be pre-determined before installation without sacrificing flowmeter accuracy. However, the Strouhal Number changes substantially at very high (i.e. 2,000,000) or very low (i.e. 20,000) Reynold's Number, R.sub.e. It also makes an apparent change when piping conditions are altered from ideal conditions, which is illustrated by small errors in measurement. In these circumstances, or any other circumstances where the Strouhal Number changes after installation, a vortex flowmeter might not be accurate.
The accuracy of vortex flowmeters also depends in part on using the proper characteristic dimension for an obstruction (i.e., a bluff body) placed in the flow path. As vortex flowmeters operate over time, these obstructions tend to erode and their characteristic dimension changes, In time, this causes vortex flowmeters to be inaccurate.
In addition, the accuracy of ordinary vortex flowmeters depends on the velocity profile of the fluid as it approaches and impinges on the bluff body. For vortex flowmeters to be accurate, it is usually required that the velocity profile be fully developed. Certain portions of piping such as elbow or expanders distort a velocity profile. Therefore, an ordinary vortex flowmeter should be installed only after many pipe diameters of straight pipe. This can be a burdensome requirement. Especially since velocity profiles can be distorted not only after pipe bends, but also when internal pipe diameters are mismatched at pipe joints, or even when the friction factor of pipe walls changes.
In general, the operation of vortex flowmeters is well known. An elongated obstruction, called a bluff body, is placed transverse to the direction of fluid flow within a conduit and generates vortices in its wake. The vortices are induced by and shed alternately from opposite sides of the bluff body. This is the Karman effect. The frequency of vortex shedding is inversely proportional to the width of the bluff body and directly proportional to the velocity of the flow, so that detecting the frequency generates signals indicative of fluid flow velocity. The measured fluid flow velocity past the bluff body, V.sub.m, is described by the equation: ##EQU1## where N.sub.s is the Strouhal Number, f.sub.m is the measured shedding frequency, and D is the diameter or width of the bluff body.
Vortices are generated in pairs, often referred to as two rows, and are disposed on either side of the longitudinal axis of the bluff body. The rotational direction of the individual vortices is such that each reinforces the other and combines with the other. As the vortices proceed away from the bluff body, the result is loss of individual character for each vortex and the creation of sinusoidal-like fluid motion transverse to the direction of the velocity of the fluid. In effect, the vortices form a standing transverse wave beyond the bluff body with the wavelength given by: ##EQU2## where V is the actual fluid flow velocity past the bluff body, and f.sub.vor is the actual shedding frequency.
The sinusoidal-like wave is persistent, with normally expected dissipation, unless disrupted by some mechanical means. In general, the strength of the vortices increases with increased velocity and with increased fluid density in the relationship of .rho.V.sup.2.
A variety of means for detecting vortices have been proposed, including the use of acoustic detection (U.S. Pat. No. 3,886,794 issued Jun. 3, 1975 to McShane), hot wires (U.S. Pat. No. 4,275,602 issued Jun. 30, 1981 to Fujishiro, et al), and a physical member located downstream of the obstruction and subject to deflection as alternating vortices pass by. In this latter approach, the physical member often takes the form of a wing and the wing may either be pivotably mounted in the conduit (U.S. Pat. No. 3,116,629 issued Jan. 7, 1964 to Bird and U.S. Pat. No. 4,181,020 issued Jan. 1, 1980 to Herzl) or be fixed to the conduit (U.S. Pat. No. 4,699,012 issued Oct. 13, 1987 to Lew, et al). In co-pending U.S. patent application Ser. No. 07/813,875, filed on Dec. 19, 1991, Vander Heyden, et al. disclose a double wing vortex flowmeter that makes flow measurements which are substantially unaffected by external vibrations.
A major shortcoming with present day vortex flowmeters is that they are sometimes inaccurate because the measured fluid flow velocity V.sub.m as determined by the measured shedding frequency f.sub.m, depends on the Strouhal Number N.sub.s, the characteristic dimension of the bluff body D, and on the fluid velocity profile; and present day vortex flowmeters do not compensate for changes in these conditions which may occur after installation. The present invention improves the accuracy of vortex flowmeters by compensating flow measurements for changes that may occur in these conditions after installation.